MANNER OF MIGRATIONS FROM SEA TO RIVERS. 45 



without apparent exceptions as pertains to their effects, and all of the exceptions have 

 not been shown 'to prove the rule.' But there are no exceptions to the fact that all 

 salmon spawn in a season of falling or fallen temperatures. Gurley was logical in reaching 

 his conclusion that the migration must be 'to-cooler' water. There are apparent excep- 

 tions to that, although logically it does not seem reasonable that a fish inexorably com- 

 pelled by heredity to spawn in cold water should enter from colder into warmer water to 

 attain that end. 



As pertains to the salmon which enter a river in summer there can be no question but 

 that in general it is in a season of rising temperature in the river and that the mean 

 temperature of the river water maj' be no colder and sometimes, no doubt, much warmer 

 than the tidal water. But there may be a question if at the actual time of entrance by 

 salmon, the river water may not be cooler for the time being. It is a well-known fact 

 that salmon do not, as a rule uninterruptedly ascend the river to the spawning places. 

 The winter and spring salmon particularly linger along the river in favorable places 

 until certain changes of conditions induce them to proceed. 



It appears as previously remarked, that temperature is not the only factor influencing 

 or governing the ascent of salmon and that many other conditions obtaining in the 

 rivers must be taken into account. Also that rivers, even in the same region may differ 

 from each other in that respect. Concerning this point, Calderwood (1907, p. 139) 

 remarked that it would be entirely erroneous to regard the conditions in Scotland as the 

 same as in a country like Norway. It may be added that it is Likely that it would be 

 fully as unsafe to conclude that conditions in New England are the same as those of 

 Newfoundland or Labrador. 



So far as temperature is concerned the foregoing evidence points to what seems to be 

 a fact that it is not the steadily lower temperature of the river water that causes the 

 salmon to enter the river and induces ascent after entering, but rather that a favorable 

 change of conditions, heralded by an influx of cooler water, takes place, thus providing 

 the 'stimulus' to which the salmon per force of heredity, is compelled to respond. Salmon 

 hiding in the estuary or pools of a river 'rise' at a certain stage of a rise in the river. A 

 rise in the river is always accompanied by a lowering of the temperature but the initial 

 stages or height of the freshet is usually accompanied by other conditions which are 

 unfavorable for the 'run.' Having started the ascent by virtue of the stimulus of cooler 

 water and other favorable conditions the salmon continues to ascend untU unfavorable 

 conditions again inhibit the ascent, to proceed again upon recurrence of a favorable 

 change, or until the end of the journey. Thus, if true, is Gurley's 'to-cooler' migration 

 substantiated. However, there still remains the mystery of the breeding catadromy of 

 some 'landlocked' salmon. 



Manner of Migrations from the Sea to Rivers. 



'It has long been a vexed question,' says Day (1887, p. 68), 'as to the manner in which 

 salmon enter estuaries and ascend rivers on their arrival from the sea, and although 

 doubtless local circumstances may occasion certain differences, still the mode of migra- 



